Field of the Invention
The present invention relates to a centerless finishing machine for finishing external surfaces of cylindrical workpieces which are fed through the machine on a continuous basis. In particular, the present invention relates to machines having pneumatic oscillators for moving finishing stones and to control devices of the machines. However, the present invention is not confined to this type of machine and may have hydraulic electromagnetic or spring-mechanized drives.
Finishing machines are for finishing external cylindrical surfaces for improving the quality of the external surface of workpieces. Finishing machines are also for shaping the radius of curvature and for adjusting these workpieces to an exact size.
The present invention is an improvement on a finishing machine described in U.S. Pat. No. 4,558,537. The disclosure of the finishing machine of U.S. Pat. No. 4,558,537 is incorporated herein by reference. This patent discloses that workpieces are supported between two drive rollers and set in rotation by these rollers. These two drive rollers rotate in the same direction. A plurality of finishing elements are arranged side by side above the rollers and the workpieces. Each finishing element consists of a finishing stone which is held by a stone holder. A piston rod connects the stone holder to a control cylinder. The finishing stone can be controlled by means of this control cylinder to move towards or away from the workpiece.
The contact pressure of the finishing stone can be varied by means of a controller for adapting the contact surfaces and to compensate for erosion of the finishing stone. The finishing stones are further set in an oscillating motion in the direction of the drive roller axes.
Tests on centerless continuous feed machines have shown that the abrasive wear on the material and therefore the quality of the workpiece, quite apart from technological parameters, e.g. the speed of the roller, the angle of the finishing stone to the workpiece, the contact pressure, is connected to the oscillation amplitude of the finishing or honing stone.
It has been further established that a change in the contact properties, between workpiece and tool (workpiece material, type of stone, number of stones used, contact pressure, length of stones) has a direct influence on the resulting oscillating amplitude. Accordingly, a problem that has arisen in centerless feed machines is to achieve a consistent standard in the quality of a particular job-on-hand. One way to solve this problem is to keep the oscillating amplitude of the machine constant, since the oscillating amplitude exerts the most influence on the finished product.
German Patent No. DE-28 14 761 describes a complex oscillation process for smoothing limited surfaces as well as the drive devices and for generating oscillation. The oscillation movements are controlled manually or via an electronically operated end switch.
German Patent No. DE-30 07 314 describes a process and a device for honing the surface of a bearing. The oscillation of the honing stones is controlled by either manual or electrical operated end switches.
German Patent No. DE-31 33 246 describes a tool machine having a range limitation device without mechanical intervention. The machine includes a potentiometer which builds up voltage corresponding to the current position of the tool. This stored voltage is compared to pre-set voltage limits in order to determine the distance limitation for the tool position.
However, the devices described in these documents permit only a rough control over the tool movements. Accordingly, the prior art devices are not capable of keeping the oscillation amplitude of the machine constant.